The invention relates to a black and white facsimile system, wherein the document is scanned line by line, the scanner signals are sampled to obtain for every line a pulse sequence of a constant number of pulses. The pulse sequences are converted into codewords for redundancy reduction. The codewords are transmitted and, in the receiver, reconverted into pulse sequences which control reproduction.
Various methods are known for reducing redundancy, in particular those exploiting only line redundancy. One of these known arts is the so-called run-length coding, whereby the successive points of equal color are counted and codewords corresponding to these figures are transmitted and, in the receiver, reconverted into pulse sequences which control reproduction (e.g. Fleckenstein et al. U.S. Pat. No. 2,909,601). Such methods enable the transmission time to be shortened considerably, but are very prone to interference. A bit error in one line falsifies the rest of the line. With unfavorable errors this may result in very conspicuous black and/or white stripes.
Systems are known in which a video signal is delayed for one line period. Normally, the signal is reproduced without delay. However, during response of a drop-out sensor, the delayed signal is reproduced (Bliss, U.S. Pat. No. 2,535,631; Holmberg, U.S. Pat. Nos. 3,347,984 and 3,366,732).